1. Field of the Invention
The invention relates to a process for producing high-molecular-weight silicone resins.
2. Background Art
Numerous publications and patent specifications disclose processes for producing organopolysiloxane resins. The syntheses generally start from chloro- or alkoxysilanes, which are hydrolyzed and condensed. In order to avoid gelling to give insoluble products, a process solvent which is immiscible with the polar aqueous medium is generally added during the synthesis. The solvent dissolves the silicone resin in a non-aqueous phase as it forms, and renders it inaccessible to the condensation catalyst, thus inhibiting gelling. Process solvents that have proven to be ideal are aromatic solvents such as toluene or xylene, technical grades of which generally comprise ethylbenzene.
In this context, reference may be made to DE 1668172 A. The synthesis disclosed uses a mixture of an organic solvent with a cosolvent, in order to obtain a homogeneous hydrolysis mixture, and the process solvent is preferably aromatic, in particular toluene. Another example of a process solvent is n-butyl acetate, and DE 1668172 A indicates that with this solvent, a cosolvent is indispensible The process is restricted to the use of alkoxysilane starting mixtures which are composed of at least 75 mol percent of CH3SiO3/2 units, and the starting silanes comprise both chlorosilanes, alkoxysilanes and alkoxychlorosilanes. The use of chlorosilane-containing mixtures in the presence of water inevitably leads to conditions which cause ester cleavage of n-butyl acetate, and during the course of the reaction it would therefore be impossible to avoid production of butanol, which by reaction with the chlorosilanes would produce unreactive butoxy-functional silicone intermediates, with the possible result that the condensation reaction would cease at low molecular weights. Stabilizing butanol additions are very conventional, specifically for this purpose, in condensable silicone resin preparations. In this connection, reference may be made by way of example to Example 20 in DE 4128893 A1, where butanol is used to adjust the solids content of a condensable siloxane preparation, and a product with stable viscosity is thus obtained. The well-known principle involved is that hydrolytic cleavage of an alkoxysilyl-Si—O—C bond becomes more difficult as the alkyl moiety becomes longer and/or more branched. DE 4128893 A1 utilizes this step to establish a condition of thermodynamic equilibrium which renders the further addition of a stabilizer or inhibitor for adjusting the storage-stability of the resultant products superfluous. Consequently, DE 1668172 A also contains no inventive example with n-butyl acetate as a process solvent.
The process of DE 1668172 A gives solids which crumble easily, with a molar mass Mw of from 1000 to 3500 g/mol, and these are naturally unsuitable for coatings and other applications requiring flexibility.
DE 854708 teaches a process for producing silicone resins via chlorosilane alkoxylation, hydrolysis, and condensation in one step, where an aromatic solvent is added as process solvent, because this helps to inhibit gelling to give insoluble products, which occurs to a particularly large extent in the inventive procedure of DE 854708.
DE 10 2005 047 395 A1 describes a multistage process for producing silicone resins from chlorosilanes by using organic solvents insoluble in water, where “insoluble in water” means that less than 1 g of solvent dissolves in 100 g of water at 25° C. and at a pressure of from 900 to 1100 hPa. However, the examples given in DE 10 2005 047 395 A1 for such solvents to some extent contradict this definition, since acetone and methyl ethyl ketone are also given as possible solvents, and the miscibility or solubility of these in water under standard atmospheric conditions is markedly higher than 1 g/100 g of water. By way of example, the solubility of methyl ethyl ketone is 270 g/l of water. The examples of DE 10 2005 047 395 A1 use only toluene, which, as the sole solvent, is known from DE 854708 to counter gelling due to uncontrolled condensation during synthesis of the resin, and the contradictory information in DE 10 2005 047 395 A1 therefore raises doubts about the usefulness of non-aromatic solvents for the synthesis of methyl- or methylphenylsilicone resins.
DE 4128893 A1 describes the synthesis of silicone resin coating compositions from alkoxy- and silanol-functional silicone resins, silanes, and siloxanes in the presence of a basic condensation catalyst and optionally with solvent or without solvent. The solvent-free syntheses in the alcohol eliminated from the starting materials do not provide high-molecular-weight silicone resins, because of the establishment of an equilibrium between alkoxylation and hydrolysis. In DE 4128893 A1, high-molecular-weight silicone resins are obtained only with xylene as a process solvent. As is obvious from the comparative examples, the process for producing high-molecular-weight silicone resins as in DE 41228893 A1 cannot be used in the solvents given in that document, other than xylene.
Aromatic solvents give cause for concern for operational, environmental, and health-related reasons. Some applications, for example bodycare products, or coating compositions, cannot use products produced in these solvents, or can use them only to a limited extent. On the other hand, aromatic solvents have excellent properties as process solvents for the synthesis of silicone resins. In particular toluene, but also xylene, is almost immiscible with water, but is a very good solvent for silicone resins. They therefore have the ability, during the condensation of alkoxy- or hydroxy-functional siloxane and/or silane precursors, to extract the silicone resins, as they form, from the polar phase into the non-polar phase, and to protect them from excessive condensation extending to gelling, and in many processes they are therefore essential in ensuring that the targeted products are successfully produced.
Apart from alcohols, polar solvents at least to some extent miscible with water can optionally be used as cosolvents in order, if necessary, to achieve homogenization of the reaction mixture or to facilitate phase transfer, and this can be a particular requirement when chlorosilane-containing starting materials are used. In the presence of alcohols, equilibria develop which, as discussed above, inhibit the formation of high molecular weights.
Further to the above, there are no examples of syntheses of high-molecular-weight, soluble silicone resins in polar solvents which are at least to some extent miscible with water, as the sole process solvent. Since the properties of the aromatic solvents are ideally suitable for carrying out syntheses of silicone resins, the expectation per se is that use of polar solvents is not possible, because the property of segregating the silicone resin from the aqueous phase as it forms, and thus protecting it from gelling, is no longer available. However, the restriction of process solvents to aromatics restricts the application range of silicone resins, and their usefulness. This is particularly relevant in the sectors of cosmetics, bodycare, food and drink, medicine, and the construction industry, where typical properties of silicone resins, such as resistance to external weathering, long life, good haptic properties, or transfer resistance would be of great benefit, but these are not used, or are subject to use restrictions, because of health-related concerns about the aromatic solvents used.